Structural Matrix Changes are Prevalent in the Corneal Stroma of Mucopolysaccharidosis Type VII (Sly Syndrome)

Abstract

Purpose: : To investigate if and how a defective enzymatic pathway involved in proteoglycan turnover affects extracellular matrix ultrastructure we performed a detailed study of a cornea obtained at keratoplasty from a 22-yr-old man with mucopolysaccharidosis type VII (MPS VII; Sly Syndrome).

Methods: : Molecular genetic analyses and assessments of enzyme activity were performed prior to surgery. After keratoplasty proteoglycan-collagen interactions throughout the stroma were studied by transmission electron microscopy and three-dimensional electron tomography. Proteoglycans were contrasted by inclusion of Cuprolinic blue in the primary fixative.

Results: : Analysis of the GUSB gene revealed compound heterozygosity for two missense mutations c.[155C>T]+[1120C>T] leading to amino acid changes p.Ser52Phe and p.Arg374Ser. Functionally, β-glucuronidase activity in isolated leucocytes was <2 % of controls. Electron microscopy revealed a morphologically normal epithelium which overlay an atypical epithelial basement membrane-Bowman’s layer. Keratocytes were packed with cytoplasmic vacuoles containing Cuprolinic blue-positive material, and three-dimensional electron tomography revealed proteoglycans in the MPS VII stroma which were much larger than in normal cornea and formed extended linkages with multiple collagen fibrils, rather than short cross-bridge connections. Collagen fibril diameter in the MPS VII stroma was less than in normal cornea and varied considerably throughout anterior (14-32nm), mid (13-42nm) and posterior (17-39nm) stroma.

Conclusions: : Cellular changes in the MPS VII cornea resemble those in other MPS, however, a diversity in collagen fibril diameter throughout the stroma and extensive interactions with supranormal-sized proteoglycan structures may be unique features of this disorder. The data suggests that an accumulation of chondroitin-, dermatan- and heparan sulphate glycosaminoglycans in the absence of ß-glucuronidase-mediated degradation can modulate collagen fibrillogenesis.

Keywords: proteoglycans/glycosaminoglycans • cornea: stroma and keratocytes • cornea: basic science